The wide-spread popularity of the Internet in both private and commercial applications has resulted in a sudden increase in the requirement for broadband data connections, the majority of which can be produced over an already existing cable so that furnishing the data line does not add to the costs involved. This technique is used by some telecommunications vendors in public networks of high bandwidth. Transmitting the user data part is done, for example, via the POTS line often already existing at the customer end. In this arrangement the user data can be transmitted in parallel to the so-called narrow-band services, e.g. digital or analog voice or fax communications.
For fast bidirectional data transmission, use is made, for example, of the asymmetrical digital subscriber line (ADSL) technology in which a modulated analog signal is transmitted via the copper pair of the POTS line. A demodulator circuit as may be a component of an ADSL modem of the customer converts the modulated analog signal into a stream of serial synchronized bits. Synchronizing the serial bit stream satisfactorily makes it necessary that the serial bit stream be permanently available. This is why, should no user data be transmitted at the time, empty data packets (so-called empty cells) are transmitted in a predefined format so that a continual data stream exists in the transmitting and receiving direction.
In achieving broadband public networks, asynchronous transfer mode (ATM) systems as standardized by the International Telecommunication Union (ITU) are finding increasing application for communicating data between subscribers. In this communication technology, the user data to be transmitted in a format in accordance with a network protocol of the local area network (LAN) is resolved into one or more ATM data cells of a fixed byte length, provided with a destination address and transmitted via packet switching exchanges to the destination address. The ATM data cells need to be converted back at the receiver end into user data in a data format in accordance with the network protocol.
In network terminology, the various protocol steps are usually termed “layers.” Involved in the case of ATM data communications as cited above are the so-called transmission convergence (TC) layer and the ATM adaption layer (AAL).
The TC layer represents a processing step controlling communication of the user data via the physical connection of the public network. In this processing step, the user data in the form of ATM cells is received as a serial data stream, checked for transmission errors, and empty cells serving to maintain synchronization are removed. When user data is transmitted, this processing step computes a checksum to be added to the ATM cells to be transmitted. If no user data needs to be transmitted, empty cells are transmitted in the direction of communication.
The AAL in ATM systems is the member linking cell-oriented transmission of the TC layer to the user data to be forwarded to a higher protocol layer. In ATM technology, various modes of communication are supported for the various applications, each of which is assigned a service type and its own data format of the interface of the AAL to a higher protocol layer. Provided for data transmission with the aid of a modem is the so-called AAL5 service type specially introduced for this application as part of the ATM specification.
The AAL handles substantially two tasks. For one thing, it provides an interface to the local area network (LAN); for another, ATM cells to be exchanged with the TC layer need to be converted into a suitable data format. This function is handled by the segmentation and reassembly (SAR) function.
Processing the functions of the TC layer and SAR function of the AAL is usually handled by a special apparatus as may be integrated in an ADSL modem, for example. It is in this apparatus that the received ATM data cells are configured and made available in a format compatible with the network protocol used (for example TCP/IP) for feeding into a LAN connecting the hardware of the net subscriber, e.g., a PC.
In previously known apparatus of the aforementioned kind, the processing functions of the TC layer and the SAR function of the ATM adaptation layer (AAL) are each implemented in two separate processing units. The two processing units may be achieved in the form of one or more processors in which various programs are handled. Since processing the data in two processing units is done with differing processing times or program priorities, a buffer storage, (e.g., a FIFO) needs to be inserted between the two processing units to hold the data until it is able to be processed by the other processing unit in each case. For reading the data in and out of the buffer storage, a separate interface to the two processing units is needed in each case. This makes the configuration of such apparatus complicated and expensive.